The present invention relates to analyte monitoring systems, and more particularly to a continuous analyte monitoring system wherein fluid is extracted from an organism and monitored outside of the organism to obtain a measurement of a characteristic of the fluid, such as analyte measurement.
Monitoring systems that sample and measure characteristics of fluids from an organism, such as a human, are well known. Many of these systems involve implanting sensors and related devices into the organism (such as under the skin) in order to obtain samples and make measurements of those samples. Even for short term implants, it has been shown that within the first several hours after implantation a rapid deposition of fibroblasts, macrophage plaques, fibrogen growth and other natural physiological encapsulation processes surround the implant and thereby impair, restrict, and modify, in a dynamic fashion, the free flow of the analytes of interest into the active sensor region of the implanted device. The typical method for compensating for these encapsulation effects involves calibrating the sensor against a conventional in vitro analysis method several times over the first few days. Further, once implanted, the sensors must be frequently calibrated, resulting in trauma to the implanted site and additional finger sticks to obtain the blood for calibration. The need to conduct multiple calibrations largely eliminates much of the advantages for many of the implanted continuous monitoring systems.
An additional restriction on the performance of the implantable sensors is that the internal environment is typically low in oxygen. This can limit the performance of many classes of reactive bio-sensors that invoke an analyte specific reaction which requires oxygen. One solution employed in some of the implants being developed is to use a restrictive diffusion membrane which limits the proportional amount of the analyte of interest which is allowed to reach the assay element, thereby extending the useable life of the implanted sensor in the oxygen lean internal environment. This compromise solution can have detrimental effects on response time, linearity of response to serum level changes in the analyte, and basic assay signal-to-noise ratio (SNR)